The invention relates to magnetic recording, and in particular to an apparatus for recording and reading data on magnetic tape in the form of a sequence of arcuate tracks which are transverse to the longitudinal axis of the magnetic tape.
The standard configuration of an information storage subsystem for a modern computer installation includes internal and direct access memory. Typically, the information storage subsystem also includes a magnetic tape drive for backup storage of information in the internal and direct access components. Two important trends in storage technology are found in the miniaturization of all storage subsystem components, and a significant increase in the information storage capacity of the internal and direct access components. The tape drive component has been miniaturized by accommodation of the quarter inch tape cartridge which has emerged as a standard in the industry. However, the storage capacity (areal density) of the tape drive has not kept pace with the increased capacities of the other storage components. Accordingly, there is an urgent need to increase the amount of information which can be recorded on a magnetic tape, which can only be realized by increasing the density of information which is stored on the tape.
Most commercially important magnetic tape drive systems are based on the reel-to-reel transport of magnetic tape past a fixed recording/reading location where a stationary single- or multiple-track head is positioned. Recording and playback in such a system is done longitudinally with respect to the tape by moving the tape on its longitudinal axis past a record/playback location where a head mechanism is located. In a stationary head tape drive, the head mechanism consists of a plurality of transversely-aligned heads which are fixedly positioned with respect to the tape during record and playback. Information is placed on the tape in the form of a plurality of parallel, longitudinally-extending tracks; the areal density of information stored on the tape is increased by reducing the dimensions of the heads and the inter-head spacing on the head mechanism. However, small head size and minimal inter-head spacing demand great precision in the manufacture of head components. As a result, the manufacturing tolerances of the tape drive, primarily the mechanical tolerances of the head assemblies, have become increasingly stringent and more difficult and expensive to achieve. Of course, the proliferation of heads is reflected in additional read and write channel electronics for each head which also adds to the expense of these drives.
As is known, in the video recording art, modern high-capacity, high-quality tape drives employ head mechanisms which rotate magnetic heads with respect to a moving tape. The high rotational speed of the "rotary head" recorders steps away from the requirement in stationary head technology for a plurality of transversely-aligned heads and associated electronics and, therefore, obviates the problems attendant with manufacture and assembly of stationary head mechanisms. Servoing is employed in the dominant classes of rotary head tape drives to align rotating heads with tracks on the tape. The servoing techniques developed for these classes of tape drives enhance head/track alignment and result in substantial reduction in track width and inter-track spacing. Consequently, rotary head tape drives enjoy a significant advantage over stationary head tape drives in areal density.
The most widely employed rotary head technology is known as transverse linear or "helical" scanning technology. In transverse linear scanning, one or more transducers (heads) are mounted on the side cylindrical surface of a head carrier drum which is rotated on an axis parallel to, but spaced from, the longitudinal path of tape travel. A succession of linear tracks is laid down transverse to the longitudinal axis of the tape. In helical scan video recorders, a tape is wrapped around a tilted drum on whose outer surface are mounted (usually two) heads. The resulting tracks are substantially straight, but have an angle to the longitudinal axis of the tape. In helical scanning, servoing information included in the tracks or in separate servo tracks is used to vary the speed of the scanner and tape in order to align the tracks with the heads.
Upon an initial consideration, helical scanning would, therefore, promise to provide an increase in areal density which would match the amplified storage capacity of the internal and direct access components of a computer storage subsystem. However, the application of helical scanning to magnetic tape drives for computer systems is limited by two significant factors. First, the tape drive mechanism must have a means for closely engaging the tape and the side cylindrical surface of the head carrier. As is known, head/tape engagement mechanisms in helical scan tape drives are large, complex, and relatively slow acting. They would, therefore, add significantly to the size of a tape drive and to the difficulty and expense of manufacture and would require a significant amount of time to change a tape cartridge. The second drawback of helical scan tape drives is that the head/tape engagement mechanism imposes a high-pressure contact between head and tape, resulting in increased wear on the head parts and decreased lifetime of tapes.
Another type of rotary head technology has been described in which the heads are mounted near the periphery of a circular planar surface and rotated thereon about an axis passing through the center of the circular surface and through the plane of a longitudinally-moving tape. This rotary head technology results in the writing of a sequence of arcuately-shaped tracks which are transverse to the longitudinal axis of the tape. Use of an arcuate scanning tape drive implies an inherently small and simple head/tape interface in which the planar circular transducer-bearing surface is brought against the plane of the longitudinally-moving tape. This interface does not require the elaborate engagement mechanisms of helical scanning tape drives in which the tape is either wound around a tilted drum or conformed to a portion of the curved surface of a straight drum. However, two significant limitations and one erroneous perception have kept this technology from being widely used. The two limitations include the lack of an adequate servoing scheme and the absence of an acceptable low-pressure head/tape interface mechanism. The misperception is that arcuate scanning provides an inherently low storage density.
Prior art arcuate scanning tape drives are described, for example, in: U.S. Pat. No. 2,750,449 of Thompson, et al; U.S. Pat. No. 2,924,668, of Hoshino, et al; U.S. Pat. No. 3,320,371 of Bach; U.S. Pat. No. 4,636,886 of Schwarz; U.S. Pat. No. 4,647,993 of Schwarz, et al; and U.S. Pat. No. 4,731,681 of Ogata. The arcuate scanning mechanism and technique described in the Thompson et al patent concerns a low speed, low density audio recorder for logging communications on two-inch wide tapes Servoing is not considered, probably because the tracks are wide, information density is low, and the signal can be tracked manually during playback. This appears to be the case as well in the Hoshino and Bach references. The rotary head recording systems of Schwarz and Schwarz et al are evidently directed to high data rate applications in which a high head rotation velocity maximizes data density at moderate tape speeds; the Ogata reference describes a magnetic recording playback apparatus in which the relatively high rotational velocity of a head with respect to a tape is used to advantage in the recording of high frequency video signals; none of these references discloses a servoing technique.
The failure of these prior art arcuate scanning references to consider servoing is significant. In fact, head track alignment in arcuate scanning is a difficult challenge because of the geometry of the arcuately scanned tracks. At the edges of the scan, the tracks converge, while in the middle of the scan there is an unavoidable divergence of the tracks. The challenge to the servoing mechanism in an arcuate scan tape drive is then to maintain a head in alignment with a track which is not linear and which does not have a constant linear geometrical relationship with adjacent tracks. The failure to provide such a servoing mechanism in the early development of rotating head technology led to the conclusion that the arcuate scanning technique was inferior to the helical scanning technique and resulted in the abandonment of arcuate scanning in favor of helical scanning for reliable, high data rate, high density recording.
In helical scanning, the head traverses the width of a tape at a shallow angle, with its travel being primarily along the longitudinal axis of the tape. Dimensional changes of as much as 0.003 inches per inch may occur in the tape due to normal shrinkage relaxation, humidity, and temperature. These changes can be corrected to a first order in helical scanning by servoing the tape tension and, thereby, the stretched length of the tape around the tilted head drum. In arcuate scanning, such a servo would be ineffective since the scan is primarily transverse to the longitudinal axis of the tape, and there is no practical method for servoing the width of the tape.
The other significant deterrent to the use of prior art arcuate scanning technology in modern computer tape drives is the great amount of wear undergone by the heads as a result of head-to-tape contact. For example, in the Thompson et al patent, a solid backing plate is compliantly urged against the tape above the rotating head. This permits very high local pressures to develop during the passage of an asperity, thereby posing a significant danger of damage to the heads and tape, and leading to head clogging. For very high density, high speed recording, such a pressure plate is impractical because intimate contact between the head and medium on the order of microinches is required, but is impossible to achieve with a solid backing plate.
Last, the common wisdom has held that the divergence of arcuate tracks results in reduced areal density because useable tape surface between adjacent tracks does not pass beneath the scanning heads and, therefore, cannot be written to or read from. Since helical tracks are parallel, they can be written with no space between them and virtually the entire surface of the tape will be used to the fullest extent possible. Thus, the assumption is that helical scanning has a higher areal density than arcuate scanning. However, helical tracks are so long that tape distortions invariably result in shape variations from track to track which limit track density and, consequently, areal density.
The inventor has also observed that shorter arcuate tracks potentially provide a higher servo sampling rate than long helically-scanned tracks, which should result in faster detection of head/track misalignment and speedier correction of head position.